Uchi uke blocks Chūdan attacks. It directs the attacks from the inside out, in the opposite direction of Soto uke.
Two important notes: The blocking arm is on the outside throughout the movement and the retreating arm is on the inside. The elbow moves horizontally to cover the entire width of the Chūdan area and to be able to execute the defense strongly.
You can find the matching book here:
The Essence of Karate